Wafer chuck with integrated reference sample

ABSTRACT

The subject invention relates to a translating wafer stage for use in optical wafer metrology instruments. The stage contains a wafer-chuck connected to translation stages for the purpose of clamping and translating the wafer so that a plurality of sites on the wafer surface may be measured. The chuck includes a holder for mounting a reference sample. The holder is movable between a retracted position where the reference sample is held below the chuck surface and an extended position where the surface of the reference sample is co-planar with the wafer surface. Therefore the holder may be installed within the area of the chuck that is utilized for wafer clamping. By this arrangement, the size of the wafer translation system can be reduced minimizing the stage travel and enabling increased spatial resolution, increased wafer throughput and reduced capital equipment and operating costs.

PRIORITY CLAIM

The present application claims priority to the U.S. Provisional PatentApplication Serial No. 60/287,360 filed Apr. 30, 2001, and Serial No.60/336,515 filed Nov. 1, 2001, both of which are incorporated herein byreference.

TECHNICAL FIELD

The subject invention relates to optical metrology devices which includea movable stage for rastering a wafer with respect to a measuring probebeam. More specifically, the invention relates to a stage which includesa system for mounting a reference chip within the footprint of the waferthereby reducing the amount of stage travel necessary to measure boththe wafer and the reference chip.

BACKGROUND OF THE INVENTION

Optical metrology instruments require periodic monitoring andcalibration. The output intensity of the light sources, the nature andextent of solarization of the optical components, chemical contaminationof the optical surfaces and the alignment of the system optics can allvary with system operating time. An instrument's performance must beregularly monitored to verify that the system continues to meetoperational specifications and that measurements are performed with therequired precision and accuracy. Frequently, this is accomplished withthe aid of a reference sample. A reference sample is awell-characterized specimen with known and temporally stable opticalproperties. Any variation in the measurement of the reference sampleoptical response is indicative of a variation in performance of theinstrument. It is the periodic measurement of the reference sample thatindicates performance problems and the requirement for maintenance orre-calibration.

Optical wafer metrology systems are characteristically configured withthe wafer surface approximately coincident with the focal plane of theoptical system. The focal plane is flat and perpendicular to the planeof incidence of the probe beam (typically defined as the x-y plane). Thevertical or z position of the wafer should coincide with the focusposition of the probing beam.

High-resolution “small spot-size” optical wafer metrology toolsilluminate a small portion of the wafer surface at the focal positionand monitor the change in one or more properties of the reflected lightcaused by the interaction with the sample surface. Characteristically,measurements are made sequentially as a translating wafer stage movesthe wafer surface “through” the illuminated region. Conventional wafer“mastering” or translation protocols include both bi-linear, x-ytranslation and single-axis translation in combination with z-axisrotation. The stage system can also include z-axis movement for raisingand lowering the wafer surface to achieve focus.

In the prior art it has been desirable to place the reference sample inthe focal plane. If the reference surface is physically located withinthe same plane as the wafer surface, no substantial refocusing of theoptical system is required during measurement of the standard sample.For systems employing x-y translation stages the reference sample istypically attached to the wafer chuck at the corner of the stage whereit does not interfere with wafer measurements. For systems employingz-axis rotation stages, restrictions posed by rotation symmetry, thelocation of auxiliary metrology instrumentation and the location anddesign of the wafer handling equipment make locating the referencesample more difficult. Even when a suitable location can be identifiedthis often requires a more expensive, long-travel stage to be used sothat the reference sample can be moved to the measurement position.These factors increase both the complexity of the instrument and itscost and size.

Accordingly it would be desirable to locate the reference sample on thewafer chuck within the wafer footprint. This offers two importantadvantages. First, the stage-travel requirements are determined solelyby the wafer dimensions. Therefore minimum form-factor wafer-translationsystems can be employed. Second, a major limitation of the prior-artapproach is eliminated permitting the use of compact wafer-translationsystems having rotary stages. In particular, the prior approach ofplacing a reference chip outside a circular chuck and connected to thechuck cannot be implemented in a rotational system where an external pinlifter mechanism is used to raise wafer. As can be appreciated, if thereference chip extended beyond the circumference of the chuck, it wouldprevent the chuck from rotating since it would intersect with the pinsof the wafer lifter. Placing the reference sample within the footprintof the chuck allows an external pin lifter to be used with a rotationalchuck.

SUMMARY OF THE INVENTION

The subject invention relates to an apparatus for holding andtranslating a wafer in an optical wafer metrology tool. The apparatusincorporates a wafer-chuck that is attached to and combined with a wafertranslation system. The apparatus further includes a holder for areference specimen. The holder is installed within the body of thewafer-chuck within the area of the chuck used for wafer clamping. Theholder is movable between a retracted position where the referencesample is below the chuck surface, and an extended position where thereference sample is substantially coincident with the wafer position.During wafer metrology the holder is maintained in the retractedposition. Measurement of the reference sample is made with the waferremoved and the holder maintained in the extended position.

Locating the reference specimen within the area of the chuck used forwafer clamping enables an economy of design. The required range ofstage-travel is set by the wafer dimensions. This admits the use ofextremely short-travel translation stages to access large wafer areas.For example, the entire surface of a 300 mm diameter wafer can beaccessed using a single 270 degree rotary-stage in combination with two±75 mm linear-travel stages, e.g. ±75 mm of travel in the x directionand ±75 mm in the y direction. The economy of motion enables increasedaccuracy of wafer positioning and increased wafer-throughput at reducedcost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the vacuum chuck with the holder inthe retracted position.

FIG. 2 is a cross sectional view of the vacuum chuck with the holder inthe extended position.

FIG. 3 is a cross-sectional view of a preferred embodiment of the wafertranslation system.

FIG. 4 is a schematic illustration of an optical metrology toolincorporating the wafer translation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 are cross-sectional schematics of a preferred embodimentof the wafer-chuck 20 showing the position of holder 50 during wafermetrology and measurement of the reference sample. FIG. 1 illustratesthe configuration of the chuck employed in the metrology of wafer 26.FIG. 2 illustrates the configuration of the chuck during measurement ofreference sample 52.

Wafer-chuck 20 includes a platform 22 for supporting and clamping awafer 26. Platform 22 includes a support surface 24 for locating andsupporting the wafer. The locating surface further includes a series ofintersecting radial and circular channels 28 which may be connected to avacuum supply via orifice 30, check valve assembly 32, manifold 34 andsupply line 36. When supply line 36 is connected to a vacuum system,surface 22, channels 28, orifice 30, check-valve assembly 32, manifold34 and supply line 36 comprise a vacuum wafer-chuck. In the preferredembodiment supply line 36 may be alternately connected to a vacuumsystem, a pressure relief-valve or a source of high-pressure gas.

The chuck further includes holder 50 for supporting and clampingreference sample 52. Holder 50 includes a reference sample 52, mountedto a spring-loaded piston assembly 54 that is free to move withincylinder 56 between a retracted and an extended position. The cylinder56 includes upper 58 and lower 60 locating surfaces. Piston assembly 54further includes a seal 64 that divides cylinder 56 into upper 68 andlower 70 hydraulic chambers. The lower hydraulic chamber is connected tomanifold 34. The upper hydraulic chamber is connected to the surface 24of platform 22 through orifice 66. A coil spring 62 is also provided tobias the holder into the retracted position.

FIG. 1 illustrates wafer-chuck 20 with holder 50 in the retractedposition, the configuration used in wafer metrology, wherein spring 62locates piston 54 at lower locating surface 60. In this positionreference sample 52 is below the surface 24 of platform 22 and wafer 26is clamped to platform 22.

FIG. 2 illustrates wafer-chuck 20 with holder 50 in the extendedposition, the configuration used for measurement of reference sample 52,wherein spring 62 is compressed and the piston 54 is driven upwards sothat the upper locating surface 58 abuts the surface of shelf 72. Inthis position reference sample 52 is located at the measurementposition, e.g. the upper surface is substantially co-planar with theupper surface of the wafer 26 as illustrated in FIG. 1.

Supply 36, manifold 34, check valve 32, piston 54, seal 64 and spring 62comprise a hydro-mechanical actuation mechanism for moving holder 50between the extended and retracted positions. Connecting supply 36 to asource of high-pressure gas causes holder 50 to move to the extendedposition. Initial pressurization of manifold 34 produces a differentialpressure across ball 40 raising the ball and pressing it against seal 38sealing check valve 32. With check-valve 32 sealed, manifold 34 andlower hydraulic chamber 70 fill with high-pressure gas. Thepressurization of lower hydraulic chamber 70, compresses spring 62raising piston 54 to the point where the piston locates at upperlocating surface 58. In this position reference sample 52 issubstantially at the measurement position, e.g. substantially the sameposition as the wafer illustrated in FIG. 1. This is the configurationillustrated in FIG. 2.

The holder is moved to the retracted position by connection of supplyline 36 to a pressure relief valve which vents lower hydraulic chamber70 and manifold 34. In this configuration, spring 62 forces the piston54 against lower locating surface 60, and the holder is maintained inthe retracted position with reference sample 52 below the surface 24 ofplatform 22. In the absence of pressurization of manifold 34, ball 40moves downward, away from seal 38 opening check-valve 32 and connectingmanifold 34 to channels 28 through orifice 30. With holder 50 in theretracted position wafer 26 can be located on surface 24 of platform 22and clamped by connecting supply 36 to a vacuum system. In thisconfiguration channels 28 are evacuated and the differential pressureestablished across the wafer 26 clamps wafer 26 to surface 24 ofplatform 22. This is the configuration illustrated in FIG. 1.

In the preferred embodiment illustrated in FIGS. 1 and 2 manifold 34 isconnected to supply line 36 through a rotary bearing assembly 74.Assembly 74 consists of a fixed hollow shaft 76 mounted in a housing 78that connects to supply line 36, and a rotary bearing 80 mounted withinthe body of manifold 34, which is fixed to platform 22. Bearing assembly74 is arranged such that a hermetic rotary seal 100 is formed betweenthe exterior surface of the hollow shaft and the inner surface of therotating manifold. In the preferred embodiment, bearing system 74 alsoserves as a thrust bearing and supports the weight of platform 22. Inthis fashion platform 22 may be rotated about hollow shaft 76 whilesupply line 36 remains fixed and connects, through bearing assembly 74,manifold 34 to a vacuum system, a pressure-relief valve or a source ofhigh-pressure gas.

It should be noted that in the preferred embodiment, a single fluid line36 is used to supply vacuum to the chuck surface to “clamp” down thewafer and to provide the pressure to raise of the reference chip. Thisdual function is important since access to the rotating stage is limitedto the rotation axis of the system.

FIG. 3 illustrates a preferred embodiment of a three-axiswafer-translation system 88 incorporating the wafer-chuck 20 shown inFIGS. 1 and 2 and described in the preceding discussion. Translationsystem 88 is comprised of wafer-chuck 20, rotary stage 86 and lineartranslation stages 82 and 84. Stages 82 and 84 are configured to providetranslation in orthogonal directions within the x-y plane. Rotary stage86 is arranged to provide rotation about the z-axis (perpendicular tothe x-y plane). In the preferred embodiment, the rotary stage has 360degrees of rotation. In addition, a mechanism (not shown) for raisingand lowering the stage system in the vertical, z-axis is provided topermit the wafer to be brought into the focal plane.

As illustrated in the FIG. 3, the use of holder 50 permits the referencesample 52 to be located within the footprint of the wafer. This allows aminimum form-factor platform to be employed with dimensions determinedby the wafer size.

FIG. 4 illustrates a preferred embodiment of the wafer translationsystem 88 incorporated in an optical metrology system 90. Opticalmetrology system 90 is configured to derive the characteristics ofsample 26 by measurement and analysis of the changes in the incidentillumination produced by reflection from and interaction with the sample26. Optical metrology system 90 includes an illuminator 92, wafertranslation system 88, sample 26, reference sample 52, detector 94 andprocessor 96.

Optical metrology system 90 may employ a plurality of measurementtechniques either alone or in combination and including detection of thechange in amplitude and the change in polarization state of the incidentillumination upon reflection from and interaction with sample 26.Further these measurements may be made using both bright-field (e.g.reflectometry) and dark-field (e.g. scatterometry) detection strategiesat a single wavelength, or at a plurality of wavelengths. Consequently,illuminator 92 and detector 94 may include one or more instrumentsselected from the group consisting of reflectometers, ellipsometers,spectroscopic reflectometers, spectroscopic ellipsometers, polarizedbeam reflectometers, polarized beam spectroscopic reflectometers,scatterometers, spectroscopic scatterometers and optical CD measurementtools. Consequently, it is advantageous to provide processor 96 toanalyze the output signals generated by the various detectors. Theseoutputs correspond to changes in magnitude, changes in polarizationstate, changes in magnitude of polarized radiation and scatter measuredat a plurality of wavelengths. The analysis protocols can treat thesignals individually or in combination to evaluate the characteristicsof a sample.

Examples of metrology tools having one or more of these measurementsystems are described in U.S. Pat. Nos. 5,608,526 and 6,278,519,incorporated herein by reference. Systems of this type include at leastone broadband light source generating a polychromatic probe beam whichis directed to the surface of the sample. The reflected probe beam ismeasured to provide both reflectometry and ellipsometric information asa function of wavelength. U.S. Pat. No. 6,278,519 also illustrates theuse of single wavelength lasers for measuring a sample.

It should be noted that reference sample 52 can be used to facilitatecalibration of the wafer stage coordinates. In particular, the locationof the edges of the reference sample can be accurately measured andcompared to stage coordinates to calibrate measurement points withrespect to a known coordinate system. In addition, measurement of thereference sample can also be used for focus adjustment in the Z-axis. Inparticular, the probe beam spot can be scanned over an edge of thereference sample while monitoring the reflected intensity. The distanceover which the intensity moves from a minimum to a maximum gives ameasure of spot size. This measurement is performed at a number ofdifferent z-positions, with the smallest measured spot size defining thefocal plane.

While the preceding discussion of the preferred embodiments has focusedon the use of a vacuum-chuck for clamping the wafer, the invention canalso employ mechanical or electrostatic means to accomplish both thefunctions of wafer-clamping and holder actuation. Furthermore,mechanical and electrostatic means can be used in place of or incombination with the preferred vacuum-clamping embodiment. In systemsemploying vacuum-clamping of the wafer the addition of hydro-mechanicalactuation may be accomplished cost-effectively. Particularly in thosecases where the required hardware, e.g. vacuum systems, manifolds,pressure relief valves, sources of high-pressure gas, etc. are alreadyincorporated in the existing wafer-clamping system. The ability tolocate the reference sample within the wafer footprint also allowsconsiderable reduction in the cost of the wafer translation systems, andthe implementation of new, high-precision translation systems at a costcomparable to existing low-precision systems. These economic benefitsaccrue from the ability to utilize lower-cost, reduced-travel stages.For example, in the preferred embodiment of FIG. 3 the entire surface ofa 300 mm diameter wafer can be measured using at least a 270° z-axisrotation stage in combination with two ±75 mm linear x and y translationstages.

What is claimed:
 1. A wafer translation stage for use in an opticalmetrology tool that permits the location of a reference sample withinthe stage area utilized for wafer metrology thereby reducing therequired translation range comprising: a platform for holding andlocating a wafer at a measurement position; a wafer motion systemcoupled to the platform for translating the platform; a holdersupporting a reference sample, said holder being located within the bodyof the platform at a location within the stage area utilized for wafermetrology, said holder being movable between a retracted position wherethe reference sample is below the platform surface and an extendedposition where the reference sample is raised to be co-planar with thewafer measurement position; and a mechanism for moving the holderbetween the retracted and extended positions.
 2. The wafer translationstage of claim 1, wherein the platform is selected from the groupconsisting of mechanical, electrostatic and vacuum wafer chuckingsystems.
 3. The wafer translation stage of claim 1, wherein the wafermotion system includes one or more elements from the group consisting oflinear translation and rotary stages.
 4. The wafer translation stage ofclaim 1, wherein the mechanism for moving the holder between theretracted and extended positions employs one or more elements selectedfrom the group consisting of mechanical, electro-mechanical andhydraulic actuators.
 5. The wafer translation stage of claim 1, whereinthe holder is a piston having a support surface for locating and holdinga reference sample, said piston being contained within a housing, saidpiston being free to move between the retracted and extended positions.6. The wafer translation stage of claim 5, wherein said housing includeslocating surfaces which limit the extent of the piston motion anddetermine the retracted and extended positions.
 7. The wafer translationstage of claim 5, wherein said holder further includes a member fixed tothe outer surface of the piston for the purpose of forming a hydraulicseal between the piston and housing, thereby dividing the housing intoupper and lower hydraulic chambers, thereby enabling hydraulic actuationof the piston motion within the housing through differentialpressurization of the upper and lower hydraulic chambers.
 8. The wafertranslation stage of claim 7, wherein the piston is spring loaded withinthe housing such that in the absence of differential pressurization ofthe upper and lower hydraulic chambers the piston locates in theretracted position.
 9. The wafer translation stage of claim 7, whereinthe piston is spring loaded within the housing such that in the absenceof differential pressurization of the upper and lower hydraulic chambersthe piston locates in the extended position.
 10. The wafer translationstage of claim 7, wherein differential pressurization of the upper andlower hydraulic chambers is achieved by evacuating the lower hydraulicchamber.
 11. The wafer translation stage of claim 7, whereindifferential pressurization of the upper and lower hydraulic chambers isachieved by pressurizing the lower hydraulic chamber.
 12. The wafertranslation stage of claim 7, wherein differential pressurization of theupper and lower hydraulic chambers is achieved by evacuating the upperhydraulic chamber.
 13. The wafer translation stage of claim 7, whereindifferential pressurization of the upper and lower hydraulic chambers isachieved by pressurizing the upper hydraulic chamber.
 14. The wafertranslation stage of claim 7, wherein differential pressurization of theupper and lower hydraulic chambers is achieved by evacuating the upperhydraulic chamber and pressurizing the lower hydraulic chamber.
 15. Thewafer translation stage of claim 7, wherein differential pressurizationof the upper and lower hydraulic chambers is achieved by evacuating thelower hydraulic chamber and pressurizing the upper hydraulic chamber.16. A wafer translation stage for use in an optical metrology tool thatpermits the location of a reference sample within the stage areautilized for wafer metrology thereby reducing the required translationrange comprising: a vacuum-chuck for holding and locating a wafer at ameasurement position; a wafer motion system coupled to the chuck fortranslating a wafer said wafer motion system including at least twoelements selected from the group consisting of linear-translation androtary stages; a holder supporting a reference sample, said holder beinglocated within the body of the vacuum-chuck at a location within thestage area utilized for wafer metrology, said holder being movablebetween a retracted position where the reference sample is below thevacuum-chuck surface and an extended position where the reference sampleis raised to be co-planar with the wafer measurement position, saidholder comprising: a piston having a support surface for a referencesample located within and free to move within a housing, said housingcontaining locating surfaces that limit the range of the piston motionand establish the retracted and extended positions; a member fixed tothe surface of the piston said member forming a hydraulic seal betweenthe piston and housing thereby dividing the housing into upper and lowerhydraulic chambers and enabling hydraulic actuation of the piston bydifferential pressurization of said upper and lower hydraulic chambers;and a hydro-mechanical actuation mechanism for moving the piston betweenthe extended and retracted position, said actuation mechanismcomprising: a conduit connecting the upper hydraulic chamber to thesurface of the vacuum-chuck; a spring arranged to locate the piston inthe retracted position in the absence of differential pressurization ofthe upper and lower hydraulic chambers and locate the piston in theextended position in the presence of differential pressurization of saidupper and lower hydraulic chambers; a gas manifold that connects thelower hydraulic chamber and vacuum-chuck to both a source of pressurizedgas and a vacuum system; and a check-valve arranged to inhibitdifferential pressurization of the upper and lower hydraulic chambersduring manifold evacuation and enable differential pressurization of theupper and lower hydraulic chambers during manifold pressurization. 17.The wafer translation stage of claim 1 utilized in an optical metrologyinstrument selected from the group consisting of reflectometers,ellipsometers, spectroscopic reflectometers, spectroscopicellipsometers, polarized beam spectroscopic reflectometers and opticalCD measurement tools.
 18. The wafer translation stage of claim 5utilized in an optical metrology instrument selected from the groupconsisting of reflectometers, ellipsometers, spectroscopicreflectometers, spectroscopic ellipsometers, polarized beamspectroscopic reflectometers and optical CD measurement tools.
 19. Thewafer translation stage of claim 7 utilized in an optical metrologyinstrument selected from the group consisting of reflectometers,ellipsometers, spectroscopic reflectometers, spectroscopicellipsometers, polarized beam spectroscopic reflectometers and opticalCD measurement tools.
 20. The wafer translation stage of claim 16utilized in an optical metrology instrument selected from the groupconsisting of reflectometers, ellipsometers, spectroscopicreflectometers, spectroscopic ellipsometers, polarized beamspectroscopic reflectometers and optical CD measurement tools.
 21. Awafer translation stage for use in an optical metrology tool thatpermits the location of a reference sample within the stage areautilized for wafer metrology thereby reducing the required translationrange comprising: a vacuum-chuck for holding and locating a wafer at ameasurement position; a wafer motion system coupled to the chuck fortranslating a wafer; a holder supporting a reference sample, said holderbeing located within the body of the vacuum-chuck at a location withinthe stage area utilized for wafer metrology, said holder being movablebetween a retracted position where the reference sample is below thevacuum-chuck surface and an extended position where the reference sampleis raised to be co-planar with the wafer measurement position; a sourceof pressure in selective communication with the holder for biasing theholder into the extended position; and a spring for urging the holderback to the retracted position.
 22. The wafer translation stage of claim21 further including a vacuum source in selective communication with theholder and also used to urge the holder back to the retracted position.23. The wafer translation stage of claim 22 further including a valvefor selectively coupling either the source of pressure or the vacuumsource to the holder.
 24. The wafer translation stage of claim 22,wherein said vacuum source is also coupled to the chuck for holding thewafer to the chuck.
 25. The wafer translation stage of claim 21, whereinthe holder is a piston having a support surface for locating and holdinga reference sample, said piston being contained within a housing, saidpiston being free to move between the retracted and extended positions.26. The wafer translation stage of claim 25, wherein said housingincludes locating surfaces which limit the extent of the piston motionand determine the retracted and extended positions.
 27. The wafertranslation stage of claim 26, wherein said holder further includes amember fixed to the outer surface of the piston for the purpose offorming a hydraulic seal between the piston and housing, therebydividing the housing into upper and lower hydraulic chambers, therebyenabling hydraulic actuation of the piston motion within the housingthrough differential pressurization of the upper and lower hydraulicchambers.